1. Technical Field
The present invention relates to a current sensor that senses charge/discharge current of a battery installed such as in a passenger car or a truck.
2. Related Art
Current sensors have been known. Such known current sensors include battery charge/discharge current sensors as disclosed in JP-A-2009-122056. The battery charge/discharge current sensor disclosed in JP-A-2009-122056 is configured to sense the charge/discharge current of a battery based on voltage across a current sensing resistor inserted between the negative terminal of the battery and a ground. In this battery charge/discharge current sensor, measured currents are summed to calculate the remaining amount of battery, and the calculated value is displayed.
Battery monitors have also been known as disclosed in JP-A-2000-194456. The battery monitor disclosed in JP-A-2000-194456 is provided with two resistors connected in series to sense current. When a normal current flow is measured with these resistors, only one of the resistors is used with the other one of the resistors being short-circuited across its ends. When a very small current flow is measured with these resistors, both of the two resistors are used.
In the current sensor disclosed in JP-A-2009-122056, the voltage across the terminals of the current sensing resistor is amplified with a predetermined gain and the amplified voltage is inputted to an analog-digital converter. It is true that, when the gain is made larger in this current sensor, the resolution is scaled up to thereby enable measurement of very small currents. However, measurement of large currents is disabled in this case because the full range is reduced. It is also true that, when the gain is made smaller in this current sensor, the full range becomes larger to thereby enable measurement of large current. However, in this case, the resolution is scaled down and hence the accuracy of measuring very small currents is lowered.
In particular, in a battery installed in a motor vehicle, the dynamic range of required charge/discharge current greatly depends on the travel status of the vehicle, as a characteristic of such a motor vehicle. For example, approximately 300 A is required in measuring current in a state of starting the engine (engine start-up status) of such a motor vehicle, while only several tens of milliamperes are required in measuring current when the vehicle is in a status of being parked (parked status). In this way, it has been difficult to measure current in all travel statuses of such a motor vehicle, and to perform accurate control of the vehicle. More specifically, an attempt to measure a current flow in a parked status using the configuration of measuring the current in an engine start-up status has resulted in insufficient resolution. Thus, it has been difficult to accurately measure current in a parked status of such a motor vehicle using the configuration for the engine start-up status.
In the battery monitor disclosed in JP-A-2000-194456, one of the resistors is required to be short-circuited across its ends using a switch in measuring large current. When this type of battery monitor is applied to the measurement of charge/discharge current of an on-vehicle battery, the reliability of the switch will not be ensured because large current flows through the starter in an engine start-up status of the vehicle. Thus, this type of battery monitor has not been applicable to the measurement of charge/discharge current of an on-vehicle battery.